Various types of polyols having different degrees of purity are commercially available. The market is consistently trending to higher purity products. The purity of polyols is affected by a wide variety of issues, including the residual metal content from the catalyst used to prepare the polyols. The ability to remove the metal content of polyols coming from the catalyst also provides the opportunity to recycle and reuse the catalyst.
Double metal cyanide (DMC) catalysts can be used to prepare, various types of polyols such as, for example, polyether polyols, polyether ester polyols and/or polyether carbonate polyols. Methods are being developed for the removal of DMC catalyst from various types of polyols to stay ahead of the demand.
Many processes have been proposed in the literature to eliminate the catalyst from polyols. The most common method is the treatment of the catalyst containing polyol with an alkali metal, hydride or hydroxide, which generates an iconic form of the catalyst and promotes agglomeration, followed by removal of the catalyst by filtration. Such methods are described in, for example U.S. Pat. No. 5,416,241 (believed to correspond to EP 0 665 254 B1) and U.S. Pat. No. 4,877,906. Another method for removal is treatment of the polyol with an oxidant such as hydrogen peroxide or oxygen containing gas, followed by filtration, as described in U.S. Pat. No. 5,099,075 and U.S. Pat. No. 5,235,114. Other examples include treatment with an acid or polymeric acid that forms chelates with the catalyst, and allows for filtration. (See U.S. Pat. No. 5,248,833, U.S. Pat. No. 6,806,348 and U.S. Pat. No. 7,678,944). Other methods of removing DMC catalysts from polyols are disclosed in U.S. Pat. Nos. 5,144,093; 5,010,047; 4,987,271; 4,721,818 and 4,355,188.
There are inherent issues or problems with the above described treatment methods. These include the need for careful control over the chemical additive to ensure that the product itself is not adversely affected. There is also the added complexity of deactivating and/or removing the added chemical itself from the polyol. In the case of polyols that contain hydrolytic groups such as with polyether carbonates, chemical treatments with compounds such as alkali metals and salts are not appropriate due to chemical attack of the carbonate linkage, which potentially damages the polyol material and leads to uncontrolled broadening of the molecular weight distribution.
Inorganic adsorbents have also been successfully used to remove DMC catalysts from polyols. Both U.S. Pat. No. 6,930,210 and U.S. Pat. No. 8,354,559 (believed to correspond to EP 2058352B1) disclose that polyether polyols can be purified such that they are high purity polyether polyols with low levels of catalytic residue, by the addition of sepiolites. Adsorbents have the advantage of being a less harsh treatment compared to the above chemical treatments, and they can be removed by filtration since they remain insoluble in the polyol.
U.S. Pat. No. 8,354,559 discloses synthetic aluminum silicate, synthetic alumina/magnesia and synthetic hydrotalcite with defined particle sizes that include >90% of the adsorbent to be less than 44 μm, were used to remove the total metal content of the polyol to less than 1 ppm. Adsorbents with larger particle sizes were found to be less effective at catalyst removal.
U.S. Pat. No. 6,930,210 describes the use of sepiolites for catalyst reduction to less than 1 ppm. Sepiolite is a naturally occurring magnesium silicate. Removal of the catalyst was successful using from 0.5 to 1 wt. % solid sepiolite.
Montmorillonite, another inorganic adsorbent, is also effective in removing DMC catalysts from polyols. Montmorillonite is less effective than sepiolite, and the filtration properties were similarly poor. The chemical makeup of montmorillonite is more complex than that of sepiolite as it contains sodium, calcium, aluminum silicate, magnesium silicate, etc. Magnesium silicate is also found in sepiolite.
Purification of polyols prepared from double metal cyanide complex catalysts are disclosed in U.S. Pat. No. 4,877,906. This method for removing the DMC catalyst uses alkali metal compounds and phosphorous compounds to precipitate the residual catalyst, which is then removed by filtration. One embodiment describes treating a propylene oxide polyol with a sodium metal dispersion, capped with ethylene oxide, treated with magnesium silicate, and then filtered through a cake of diatomaceous earth filter aid to remove at least a portion of the catalyst. The catalyst removal is then substantially completed by treating the polyol with hypophosphorous or phosphorous acid to precipitate the remaining solubilized double metal cyanide complex catalyst residue, neutralizing the excess acid with magnesium silicate and filtering the polyol again.
An advantage of this invention is the selection of a specific grade range of diatomaceous earth allows for adsorption of the catalyst while maintaining unique improved flow rates. It is believed that this is due to the porous nature of the biogenic silica in diatomaceous earths as compared to the affective natural clays in sepiolite and other natural adsorbents. Diatomaceous earth also has a higher purity with regard to silica content and greater composition consistency than the clays. The literature reports that sepiolite typically has a silica range of 58 to 75% with the average of 68%. See “Developments in Palygorskite-Sepiolite Research: A New Outlook on these Nanomaterials”, by A. Singer and E. Galan; Vol. 3; Elsevier: UK, 2011; p. 38. By comparison, the silica range of diatomaceous earths is typically from 80 to 90%. See “Diatomite: U.S. Geological Survey Mineral Commodity Summaries” 1998, by L. E. Antonides; p. 56-57.